The preferred embodiment relates to a method for testing the functionality of a field device for an industrial process as well as to a field device for an industrial process.
A field device for an industrial process is to be understood as a control device which sends control signals to a final controlling device of an industrial processing plant. A final controlling device can for example be comprised of an actuator and a control valve actuated by the actuator, the control valve being often used for controlling a stream of a fluid in an industrial processing plant. Due to strict safety regulations, especially in chemical processing technology such as petrochemical technology, or in power technology such as nuclear technology, it is to be assured that the final controlling device moves the control valve into a safe position causing the control valve for example to completely open or closed in order to react to an emergency situation of the plant. Generally the final controlling device is operated by means of a secondary power such as electricity, or hydraulic or pneumatic energy.
The preferred embodiment concerns especially a field device for an industrial process designed as a positioner or position controller for sending an electrical control signal to a final controlling device comprised of a pneumatic actuator and a control valve positionable by means of the actuator.
The safe position the device is supposed to adopt in case of failure of the secondary power or reception of a certain safety control signal, is realized by means of the pressure of preloaded springs in the case of a pneumatic actuator connectable to a constant air pressure supply of for example 6 bar as a secondary power supply. The springs release their potential energy as soon as the pneumatic actuator is separated from the air pressure supply by the field device and is vented. In this case venting the pneumatic actuator corresponds to the pneumatic safety control signal sent by the field device to the pneumatic actuator.
In a simplified embodiment, a field device used for sending control signals to a pneumatically operated final controlling device can be an I/P converter which usually generates standardized pneumatic output signals between 0.2 to 1 bar or 0.4 to 2 bar as a function of the received electrical control signal for controlling the pneumatic actuator correspondingly. For adopting the safe position for pneumatic actuators it is necessary to vent the latter, which means to lower the output signal of the I/P converter to nearly 0. Commonly, this will occur when the I/P converter receives an electrical zero-signal or at least a signal below a specific threshold value.
A field device designed as a position controller having a pneumatic actuator is known from DE 44 29 401 C2. The field device comprises an I/P converter as a so-called pre-control step. A safety circuit formed by a relay is provided between an electrical circuit of the position controller and the I/P converter along the electrical control path and interrupts an electrical control line to the I/P converter in case of an operational error. In this way, the I/P converter adopts the operational safety state and vents the pneumatic actuator. The electrical circuit of the position controller is continuously supplied with power through a standardized 4 to 20 mA control signal according to the so-called “Live-Zero”-principle. Thus, the pneumatic actuator can be vented by means of the relay safety circuit even while the position controller receives a set-point signal significantly higher than 4 mA. An electric power supply for the electrical circuit of the position controller is always guaranteed, even if the field device is commanding the safe position be means of venting.
DE 10 2005 024 686 A1 describes a position controller with an electrically operable safety switch immediately preceding the I/P converter. The function of the safety switch is to enable a safe withdrawal of power from the I/P converter. Preferably, the safety switch is comprised of high reliability electronic components and receives the input signal from the position controller. If the voltage or current of the input signal is above a predetermined value such as 4.5 mA or 4 mA then the I/P converter receives a control signal from the micro processor of the position controller. If the voltage or current of the input signal drops below the predetermined value, the power input of the I/P converter will be cut by the safety switch, which will cause the pneumatic actuator to vent.
The established structure for a position controller according to DE 10 2005 024 686 has the advantage to enable safety certification relatively simply. From a safety technological aspect, the position controller is reduced to the safety switch, which is composed of reliable and easily verifiable electronic components. A certification of a microprocessor for the position controller would be much more difficult, because the failure probability of a microprocessor depends on many operational circumstances.